Osmosis and Tonicity •The body is mostly water •Total body water•Age and sex influence the total body water content •Less variation of water between compartments Osmosis is the movement of water across a membrane –Occurs in response to a solute concentration gradients –Movement through the membrane, aquaporins, and water-filled ion channels Tonicity Describes the Volume Change of a Cell Osmolarity: Describes the Number of Particles in Solution; the concentration

–Tonicity depends on the concentration of nonpenetrating solutes The relative concentration of nonpenetrating solutes in the cell and in the solution determines tonicity  Penetrating solutions contribute to the osmolarity of a solution but NOT to its tonicity.

-Osmotic equilibrium occurs because water moves freely between most cells and the extracellular fluid -Water will continue to move across membranes into more highly concentrated compartments until the concentrations (solute/volume) are equal, hence osmotic equilibrium -Osmotic equilibrium does not take into account what particles are present in each compartment, just the total number.  -the key is that water moves freely but the solutes do not. -Na+ and Cl- are more highly concentrated in the ECF while K+ and many anions are more highly concentrated inside the cell. -Each ion is in chemical disequilibrium because it is not evenly distributed between the two compartments. -Although the compartments are chemically different (chemical disequilibrium), when all solutes in one compartment are compared to all the solutes in another compartment they have the same total concentrations of solutes (are in osmotic equilibrium).

Compare: They all have to do with concentrations Contrast: Molarity - moles/1 L solution Molality - moles/1 kg solvent Osmolarity - osmoles/1 L solution Osmolality - osmoles/1 kg solution *These involve one solution Tonicity - hypo, iso, hyper *These involve two solutions w/ a membrane   molarity: measures the number of moles of solute per unit volume of solution concentration of a solution expressed as the number of moles of solute per litre of solution osmolarity: measures the number of osmoles of solute particles per unit volume of solution. osmolality: concentration expressed as osmoles of solute per kilogram of water

Step 1: Assume that all intracellular solutes are nonpenetrating. Step 2: Compare osmolarities before the cell is exposed to the solution. Step 3: The Tonicity of a solution describes the volume change of a cell at equilibrium Step 4: Determine tonicity by comparing nonpenetrating solute concentrations in the cell and the solution. The net water movement is into the compartment with a higher concentration of nonpenetrating solutes. Step 5: Hyperosmotic solutions are always hypotonic.